Memories can be passed down to later generations through genetic switches that allow offspring to inherit the experience of their ancestors, according to new research that may explain how phobias can develop.

Scientists have long assumed that memories and learned experiences built up during a lifetime must be passed on by teaching later generations or through personal experience.

However, new research has shown that it is possible for some information to be inherited biologically through chemical changes that occur in DNA.

Researchers at the Emory University School of Medicine, in Atlanta, found that mice can pass on learned information about traumatic or stressful experiences – in this case a fear of the smell of cherry blossom – to subsequent generations.

The results may help to explain why people suffer from seemingly irrational phobias – it may be based on the inherited experiences of their ancestors.

So a fear of spiders may in fact be an inherited defence mechanism laid down in a families genes by an ancestors' frightening encounter with an arachnid.

Dr Brian Dias, from the department of psychiatry at Emory University, said: "We have begun to explore an underappreciated influence on adult behaviour – ancestral experience before conception.

"From a translational perspective, our results allow us to appreciate how the experiences of a parent, before even conceiving offspring, markedly influence both structure and function in the nervous system of subsequent generations.

"Such a phenomenon may contribute to the etiology and potential intergenerational transmission of risk for neuropsychiatric disorders such as phobias, anxiety and post-traumatic stress disorder."

In the study, which is published in the journal of Nature Neuroscience, the researchers trained mice to fear the smell of cherry blossom using electric shocks before allowing them to breed.

The offspring produced showed fearful responses to the odour of cherry blossom compared to a neutral odour, despite never having encountered them before.

The following generation also showed the same behaviour. This effect continued even if the mice had been fathered through artificial insemination.

The researchers found the brains of the trained mice and their offspring showed structural changes in areas used to detect the odour.

The DNA of the animals also carried chemical changes, known as epigenetic methylation, on the gene responsible for detecting the odour.

This suggests that experiences are somehow transferred from the brain into the genome, allowing them to be passed on to later generations.

The researchers now hope to carry out further work to understand how the information comes to be stored on the DNA in the first place.

They also want to explore whether similar effects can be seen in the genes of humans.

Professor Marcus Pembrey, a paediatric geneticist at University College London, said the work provided "compelling evidence" for the biological transmission of memory.

He added: "It addresses constitutional fearfulness that is highly relevant to phobias, anxiety and post-traumatic stress disorders, plus the controversial subject of transmission of the ‘memory’ of ancestral experience down the generations.

"It is high time public health researchers took human transgenerational responses seriously.

"I suspect we will not understand the rise in neuropsychiatric disorders or obesity, diabetes and metabolic disruptions generally without taking a multigenerational approach.”

Professor Wolf Reik, head of epigenetics at the Babraham Institute in Cambridge, said, however, further work was needed before such results could be applied to humans.

He said: "These types of results are encouraging as they suggest that transgenerational inheritance exists and is mediated by epigenetics, but more careful mechanistic study of animal models is needed before extrapolating such findings to humans.”

It comes as another study in mice has shown that their ability to remember can be effected by the presence of immune system factors in their mother's milk

Dr Miklos Toth, from Weill Cornell Medical College, found that chemokines carried in a mother's milk caused changes in the brains of their offspring, affecting their memory in later life.